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Sensor Types
A sensor measures some kind of excitation/input and provides
an output/quantity of what is being measured. Transducers take this measured quantity and convert it to a more
convenient unit for interpretation. Transducers are further
categorized as either passive or active. Passive transducers
generate their own power from the measurand, meaning they
do not need an external power source. Compared to active
transducers, which do rely on external power, passive transducers tend to be simple, more reliable and less expensive.
However, passive transducers tend to distort/load the measurand more than active sensors. In the literature, the terms
sensor and transducer are used interchangeably, and thus the
following measurement tools are referred to as sensors [45].
The following section describes the different types of PS
that can be used to monitor individuals at home. A comparison
of the advantages and disadvantages of each type of pressure
sensor is shown in Table 1.

Load Cells
Load cells are transducers that convert an applied force into a
digital quantity depending on what kind of load cell is used.

For example, strain gauge load cells convert the applied load
into a resistive change in the material which can then be converted to a voltage output through a Wheatstone bridge. The
results can be converted into a weight measurement, usually
via a calibration procedure (Fig. 3a).
Load cells do not directly measure pressure, but they have
been used to measure similar health related measurements in
the literature. Typically, they are used to monitor larger movements in the smart-home environment. Gaddam et al. and
Zahradka et al. used load cells placed below the bed legs (Fig.
2a) to classify the position of an individual in bed with an accuracy of 74.9% and movement shift with an accuracy of 79.7%.

Flexiforce Sensors
Flexiforce sensors are small, lightweight, low power force sensors that can be arranged in a variety of patterns to measure
weight applied, usually through a resistive change in the sensor materials caused by the force (Fig. 3b). These sensors have
been placed under the legs of a bed (Fig. 2a), a chair and under the toilet seat in [4] to monitor ADL. These sensors are also
the underlying technology for the Tekscan pressure mats that
is described in the following section [54].

Table 1 - Pressure sensor comparison
Sensor Type

Advantages

Disadvantages

Conductive-ink

** Very sensitive
** Flexible

** Expensive
** Low durability of the ink

Piezoresistive and
Piezoelectric

** Moderately sensitive
** Good linearity
** Unobtrusive
** Thin
** Flexible

** More rigid
** Higher levels of pressure hysteresis
** High temperature sensitivity

Air-pressure

** Commercially available
** Good for breathing detection

** No position information
** Limited spatial information based on air cell size

Capacitive

** Very sensitive
** Low levels of pressure Hysteresis
** Low temperature sensitivity

** Generally non-linear
** Sensitive to temperature
** Measurement drift

Inductive

** Linear
** Low pressure hysteresis
** Low temperature sensitivity

** Moderate sensitivity

Load cells

** Linear
** Simple

** Measures applied force rather than pressure, not much
force distribution information
** Rigid
** Not good for providing spatial information

Electromechanical
film sensors

** Good for transient signal monitoring
** Robust to noise

** Does not do well for long term static monitoring

Fiber-optic

** Low Cost
** Unobtrusive

** Degrade with long term use
** Non-linear
** Sensors are large in size

Proximity sensors

** Customizable

** Require a compressible layer/material

Air sensors

** Good for transient signal monitoring

** Poor static measurements
** No localization information
** Expensive and sometimes integrated into bed mattress

16	

IEEE Instrumentation & Measurement Magazine	

April 2021



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